You may need an architect and/or engineer to help you with this. My guess is the best thing you could do is to tear out the old stuff and go a little deeper. There is a great benefit to having a basement with accessible space between the ceiling and the floor above. That means at least nine foot high foundation walls, probably higher. If you post the location, someone familiar with the jurisdiction could may be able to offer more concrete (pun intended) guidance. My wild guess DC suburbs. How'd I do?

Chisue, I would have agreed about the basement thing but here in my new location basements are the exception not the rule. At least when it comes to ranches which were mostly built in a specific time period. I've been told that there is a water table problem here but when I reply new construction seems to have more basements than existing - well they know how to build basements better now. ????? Correspondingly this is the only area I've ever lived in the Midwest where there are actually areas in the country that people have water trucked into cisterns because there isn't any well water - so.....what's with the water table again? I was also told that back during the 50s when ranches were popular that builders convinced people that basements were a always a problem and it was better for resale not to have one. Probably 7 out of 10 ranches will be on a slab. For whatever reason Cape Cods (a style built slightly earlier will more often have a basement. I have heard that older farmhouses with cellars often do have water issues. This place is confusing to me, and I sometimes think this area had just been taken in by the construction industry but that's a thought and is probably not correct. I consider Dayton midwestern but sometimes it seems more...appalachian (not trying to be disrespectful) One realtor I talked to on the phone, when asking about the whole basement issue here actually seemed a little confused and made a comment to the effect of 'houses with basements?" like she had never heard of it. I know that's probably not what she meant entirely but it points out that people here expect or are certainly more accepting of slabs or crawlspaces, basements are more of a bonus, if dry anyway. As for pricing - ranches with basements are generally 20,000-40,000+ more which is closer to 15-20% more than ranches on slabs. And while I realize it doesn't matter because its all about existing markets, I just got to wondering - do basements cost 40,000? How about a crawlspace vs. a slab? Ive lived most of my live n the Midwest, always in a house with some sort of basement. Personally I won't live on a slab, and am not thrilled with the idea of a crawlspace, which is why I'm still in an apt looking.

"Basements Vs slabs", Practically all basements have slabs and most of them have plumbing underneath, same for 99.99% of commercial buildings. Not building a slab (or basement) for plumbing fears is typical for DIY types unfamiliar with concrete work. Designed and built right, there should be no reason to tear up the concrete in one's lifetime. If it does need to be redone, it's still light construction and not a problem for professional contractors. Building a basement or crawlspace below variable water tables can be appropriate if there is enough topography to daylight the drainage. Relying on pumps is a recipe for eventual failure. The wetter the site, the bigger the concern. Daylighted drainage that works by gravity, should be designed well, with plenty of clean gravel, silt protection and cleanouts. If your builder or grader shows up with corrugated, single wall PE, better call in an experienced civil engineer. It sounds like your lot is ideal for a stemwall slab, basically a crawlspace filled with compacted fill or better yet clean gravel and topped with a slab. This elevates the slab and provides gravity proven drainage below. Worthy's FPSF suggestion is also worth a look if your lot is really that flat. Slabs have better building science fundamentals being a well defined boundary (drainage, air barrier, insulation) between the elements and living space. Crawls are more confused. Vented crawlspaces in humid climates (east of MS) should be illegal, and if you decide to go unvented, ensure the entire crawl floor slopes to a daylighted drain. With a good design team, forced-air ductwork does not have to go in a basement, crawlspace or attic.

Here's the thing you have to remember about relative humidity: when you cool air, relative humidity goes up. Yeah, air conditioners reduce humidity, but they do it by dropping the air to below its dew point, at which time the humidity condenses out of the air and becomes that condensation that drains away. The air emerging from the vents on your a/c unit is darn close to 100 percent relative humidity for its temperature, maybe 55-60 F or whatever. The lowering of relative humidity comes as a result of this newly-cooled air mixing with warmer room air and having its temperature go up. The other way to dehumidify air is simply to heat it. That's why the relative humidity goes down in the afternoon and you see those 90 - 100 percent relative humidity readings early in the morning when the temperature is at the day's lowest. This is also why a dehumidifier first cools the air to below its dew point and then warms it back up by routing the air first through the evaporator then its condenser. If you only cool the air (and then throw the heat away outdoors, as an a/c unit does) you're only able to dehumidify to the extent that you can continue to cool the air. You take a cool damp basement that's only maybe in the mid-60s to 70 temperature-wise, start running an air conditioner there that's getting rid of heat, and pretty soon you can't remove anymore moisture because your air temperature is getting colder (naturally increasing the relative humidity). And, the air emerging from the a/c is now only a few degrees cooler than the air going in, so there isn't much moisture being removed there either. This same thing occurs if it's a damp day and you run a/c to dehumidify the house. Because you're only cooling the air, it can only be dehumidified to the extent that it, a) needs to be cooled, and b) can increase in temperature after emerging from the a/c system. The bottom line is that precise control of indoor relative humidity is hard with normal mechanical equipment that's available to consumers. But here are a few things to keep in mind: First, a dehumidifier is good only where you can use some heat. They're going to add to the air an amount of heat exactly equal to the power they use, so if you buy a 900-way dehumidifier it's going to add about 3,000 btu of heat to your space. Do you want that heat? If you're also running air conditioning probably not. In cool, weather, or in a place that's normally damp such as a basement, yes - the heat is wanted and it also helps dehumidify. Second, the best way to control humidity with air conditioning systems is with low-capacity equipment running a high percent of the time. You can do this with a variable-capacity compressor, or if you want to do it on the cheap you can supplement your central a/c with a 5,000 btu window unit running constantly. (Combined with raising the thermostat setting of the main system so the window unit does more of the cooling, especially at high-humidity, lower-temperature times such as nights and rainy days.) The key is, keep removing moisture while cooling less.